When components are being coated, it is not always the case that the entire outer or inner surface of a component is to be coated. This is true of all coating processes, such as for example plasma spraying, PVD (physical vapor deposition) or CVD (chemical vapor deposition) processes.
Parts of the coating where coating was not desired have to be removed. This often has to be done by abrasive removal of the coating in these regions, since the material of the coating bonds very firmly to the component or even reacts therewith if, for example, the component has been coated with aluminium in order to carry out alitizing.
However, the abrasive removal of coating material often leads to a certain amount of damage to the component or to a certain change in its geometry. This is the case, for example, with sand-blasting, in which the sand particles remove, for example, parts of an aluminium coating from the component.
U.S. Pat. No. 6,036,995 demonstrates the need for masking by adhesive tapes, metal foils in order to cover surface regions which are not to be coated. The coating is carried out by application of slurry. Metal foils do not bond securely to bases and therefore offer only insufficient protection, for example in an evaporation-coating installation in which the particles which are to be applied move in all directions (and thereby migrate beneath the foil).
Adhesive tapes are unable to withstand high temperatures.
In U.S. Pat. No. 5,985,368, coating is carried out using a ceramic slurry. No masking takes place.
U.S. Pat. No. 6,283,714 demonstrates the need for masking for coatings comprising aluminium. However, a specific arrangement inside the coating installation prevents aluminium from being deposited on the undesired locations.
GB 2 210 387 a describes a mask being applied by means of a slurry or paste containing a binder.
JP 11 181 561 discloses a masking, the masking consisting of an organic solvent, an organic binder and a powder.
This is a general procedure. Powder with binder in a carrier medium is applied to a component. The binder holds the powder together, is burnt out after the application and evaporation of the carrier medium and the powder is sintered so that it remains stable.
Components as described above are not only coated, but also provided with holes. Particularly turbine blades or vanes of gas turbines comprise film cooling holes whereby cooling air introduced into the blade or vane can exhaust and therefore cool the blade or vane.
Such holes are drilled into the blades by irradiating the surface of the component with an energy beam like laser beam. DE 100 63 309 A1, U.S. Pat. No. 6,054,673 and U.S. Pat. No. 6,172,331 B1 demonstrate examples of methods for drilling holes into turbine blades. However, it was detected, that the drilling operation is accompanied by deposition of material melted by the energy beam onto the surface of the component particularly adjacent to the hole. Hence, it is necessary to remove this material from the surface by an additional method step which is expensive and might destroy the coating locally.